Radiant electric heaters are known in which an element of coiled bare electric resistance wire is supported on, and secured by staples to, a layer of microporous thermal and electrical insulating material compacted in a metal support dish. Such heaters are described, for example, in GB-A-1 580 909 and are incorporated in glass-ceramic smooth top cookers.
The term `microporous` is used herein to identify porous or cellular materials in which the ultimate size of the cells or voids is less than the mean free path of an air molecule at NTP, i.e. of the order of 100 nm or smaller. A material which is microporous in this sense will exhibit very low transfer of heat by air conduction (that is collisions between air molecules). Such microporous materials include aerogel, which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel were dried directly from a liquid. A substantially identical structure can be obtained by controlled precipitation from solution, the temperature and pH being controlled during precipitation to obtain an open lattice precipitate. Other equivalent open lattice structures include pyrogenic (fumed) and electro-thermal types in which a substantial proportion of the particles have an ultimate particle size less than 100 nm. Any of these particulate materials, based for example on silica, alumina or other metal oxides, may be used to prepare a composition which is microporous as defined above.
The microporous insulation typically comprises a dry particulate microporous material as defined hereinabove mixed with ceramic fibre reinforcement, titanium dioxide opacifier and, for high-temperature use, a small quantity of alumina powder to resist shrinkage. Such insulation material is described in GB-A-1 580 909.
Radiant electric heaters have also been proposed in which, instead of an element of coiled resistance wire, an element comprising an elongate electrically conductive strip of a metal or metal alloy is provided, the element being supported on edge on an insulating base. Arrangements of this kind are described, for example, in U.S. Pat. Nos. 600,057, 3,612,829, 3,991,298, 4,161,648 and 4,292,504. In U.S. Pat. No. 600,057, a conductor is mounted on a metal support, or in a groove formed therein, by means of a coating of insulating material such as a vitreous enamel. In U.S. Pat. No. 3,612,829, a convoluted conductive strip element in the form of a spiral is located in recesses pre-formed in the surface of a cast or molded fibrous ceramic refractory material. Staples are used to secure the strip element to the supporting base. In U.S. Pat. No. 3,991,298, the conductive strip element is in the form of a spiral and is loose fitted in a pre-formed spiral groove in a rigid base of fire-resistant mortar.
In U.S. Pat. No. 4,161,648, a convoluted strip element of spiral form is provided with integral downwardly-extending mounting tabs which penetrate an electrically insulating sheet of high-temperature-withstanding board material. In the case of a thin sheet of board material, the mounting tabs are bent over at the back of the material. The board-like insulating sheet with the element thereon is then located on top of a layer of microporous thermal insulation material in a supporting dish. In the case of a thick sheet of board material, a hardenable substance is used and is hardened after the tabs have been urged into the material.
In U.S. Pat. No. 4,292,504, a heating element in the form of a thin, foil-like strip of expanded metal is supported on edge substantially along its entire length in a serpentine groove formed in the upper surface of a ceramic fibreboard. The heating element is cemented or held by friction in the groove formed in the board. It will be appreciated that a surface having a groove formed therein is not a substantially continuous surface.